The present invention relates to a new transcoder rate controller for a data transmission network, in particular the GSM network, and to a new method of transmitting data between switching controllers in a data transmission network.
Data transmission networks generally require transcoders to transcode data from one format/data rate to another format/data rate. One such data transmission network is the GSM network.
FIG. 1 of the accompanying drawings is an exemplary illustration of a standard GSM network arrangement.
As can be seen, in a GSM network arrangement a mobile station (MS) 1 is connected to a base station sub-system (BSS) 2 which provides and manages transmission paths between the mobile station (MS) and the GSM network switching sub-systems. In this case, the base station sub-system (BSS) 2 is connected to a mobile switching controller (MSC) 3. The mobile switching controller (MSC) 3 controls the routing of the signal between the mobile station (MS) 1 and another mobile switching controller (MSC) via the inter-MSC E-channel, to the Public Switched Telephone Network (PSTN) or to another base station sub-system (BSS) connected to that mobile switching controller 3 (not shown). Typically, a mobile switching controller (MSC) 3 is connected to a number of base station subsystems (BSS) 2 providing coverage for a large number of mobile stations (MS) 1.
The base station sub-system (BSS) 2 provides several different functions, which are shown in FIG. 1 as separate blocks within the base station sub-system (BSS). Firstly, a base transceiver station (BTS) 4 is provided which handles the radio communication with the mobile station (MS) 1. A base station controller (BSC) 5 controls the operation of the base transceiver station (BTS) 4. Finally, a transcoder rate controller (TRC) 6 performs GSM specific speech encoding and decoding and rate adaption for data.
The base transceiver station (BTS) 4 is connected to the base station controller (BSC) 5 by way of the A-bis channel and the base station controller (BSC) 5 is connected to the transcoder rate controller (TRC) 6 by way of the A-ter channel. The A-bis and the A-ter channel data rates are variable, and depend on the coding rate of the data received from the mobile station (MS) 1. As is well known, a number of different data rates for the speech signal received from the mobile station (MS) 1 have been specified in the GSM standard. However, the data rate is typically 4xc3x9716 k for full rate/extended full rate coded speech or is 8xc3x978 k for half rate coded speech.
The use of Tandem-Free Operation for GSM speech has recently been standardised and is outlined in ETSI specification GSM 08.62. The use of Tandem-Free Operation (TFO) allows communication via in-band signalling between transcoders.
It is desirable to reduce the data rate of signals sent between adjacent mobile switching controllers (MSC) 3 on the E-channel to minimise costs. Previously, digital circuit multiplication equipment (DCME) has been provided to compress data on the E-interface. FIG. 2 shows a diagrammatic representation of a previous arrangement in which a digital circuit multiplication equipment (DCME) 7 is connected to the mobile switching controller (MSC) 3 to compress the E-channel data. The digital circuit multiplication equipment (DCME) 7 provides speech compression and allows a number of speech channels to be combined into one time slot. At the other end of the compressed E-channel interface a further digital circuit multiplication equipment (DCME) 8 decompresses the received compressed E-channel data and provides the decompressed E-channel data to its associated mobile switching controller (MSC). The mobile switching controller (MSC) 9 can forward the data on to the base station sub-system 10 corresponding to the mobile station 11 for which the data is intended.
The transmission saving provided by the use of digital circuit multiplication equipment (DCME) 7, 8 in previous arrangements is achieved at the expense of speech quality, thus resulting in an undesirable tradeoff between speech quality and transmission efficiency within the transmission network. Furthermore, the provision of a separate digital circuit multiplication equipment (DCME) 7, 8 within the E-channel between adjacent mobile switching controllers (MSC) 3, 9 undesirably increases the operation and maintenance requirement of the network. The increase in operational costs as a result of the maintenance requirement of the separate digital circuit multiplication equipment (DCME) can outweigh the benefit gained by using the digital circuit multiplication equipment (DCME).
The present invention therefore seeks to optimise both speech quality and transmission efficiency within a data transmission network whilst reducing the operation and maintenance overheads.
In accordance with the invention, therefore, the E channel data output from the mobile switching controller is routed through the transcoder rate controller. The use of the transcoder rate controller in accordance with the invention results in improved speech quality together with band width compression, without a significant additional operation and maintenance requirement.